Hi Katt! Welcome to the forum!
Firstly, I'm sure you're going to look stunning for your formal next year regardless of if you reach your 20kg goal (which is definitely doable btw, as long as it's a realistic target)!
Secondly, you will still look just as stunning if you lose some weight, you'll just be a bit healthier too!
I think it's great you're getting healthy and doing it while you're young so you can make the most from your youth (listen to me, I sound like a grandma!) Just try not to get too much of your self image bound up in your body image.
As for motivation, start with little steps that change what you're doing in small ways. Huge changes are very difficult to maintain. Using a food and exercise diary will help give you some good info to make smart, sensible changes with.
35kg lost. (November 2005 - October 2006)
15kg gained again (as at October 2010).
Back to the drawing board - Let's do this thing!
"You can't change the winds, but you can change the sails"
"Reach out and take control of what lands in your lap"